1. Field of the Invention
The present invention relates to a system and method for communication between a plurality of networks through a NAPT (Network Address Port Translation).
2. Description of the Related Art
The use of a router having a NAT (Network Address Translation) or NAPT (Network Address Port Translation) function enables a plurality of terminals to be connected to a global network using a single global address. On the other hand, when IP telephone or video communication is performed principally in local environments using the network, all the addresses used in such environments are local addresses.
Some of these communication systems exchange their addresses in data using a unique style such as SIP, in addition to the IP's basic field indicating the source and destination. If such a system is expanded to reach a destination beyond the global network, the local address will be contained in the data returned by a terminal receiving the communication since the contents of the data will not be changed by the NAT or the like.
In order to prevent this, several methods are proposed. For example, the NAPT device itself analyzes the contents of data to rewrite the address contained in the data. Alternatively, the terminal itself previously designates the global address of the NAPT device when performing communication.
However, these methods have problems, including the need of modification of the related devices, and heavy load imposed on the NAPT devices.
One of these technologies is described in Japanese Laid-Open Patent Publication No. 2005-45678. This prior art technology provides a communication method which enables direct communication between hosts without altering the NAT function of a router. Specifically, this communication method uses a first router, a first host connected to a network through the first router, a second router, a second host connected to the network through the second router, and a controller connected to the network. The first and second routers have the NAT function and the masquerading function.
Another related technology is described in Japanese Laid-Open Patent Publication No. 2004-304318. The purpose of this technology is for easy communication with other devices through a NAPT device. Specifically, a local communication device uses a service provision device for devices to acquire communication source address port information that is global address port information of a relay device, and exchanges the communication source address port information with a local communication device which it is communicating with. Based on the communication source address port information, the local communication device exchanges an opening request packet by a plurality of methods to search a port openable for communication, and opens and maintains the communication by using an opening/maintenance packet.